Page 84 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

FORCE

FORCE

Figure 5-9. A Couple Force
The two forces are acting 180 degrees out of phase and in
different planes, causing the shaft to wobble about its centerline.
Note that the correction weight could also be a second source of
unbalance and that it would not necessarily have to be directly
opposite the unbalance nor equal in weight.
To prevent the formation of a couple force, the correction
must act in the same plane as the unbalance, or resolve to the
same plane. That is, the sum of the forces and the sum of the
moments must equal zero. A moment is a force times the distance
from a reference point.
In Figure 5-10 it can be seen that splitting the correction
weights into two equal parts and spacing them equal distances
from the unbalance is an acceptable solution to an unbalance.
However, the number and location of the correction weights must
always meet two basic criteria.

FORCE FORCE

FORCE FORCE FORCE

Figure 5-10. Two Acceptable Solutions to an Unbalance